nvidia-optimus: blacklist "nvidia-uvm" (#396074)

Even though other NVIDIA modules are blacklisted, `nvidia_uvm` is loaded and the NVIDIA card is still in use; In order to completely disable NVIDIA graphics we need to blacklist the `nvidia_uvm` kernel module

nixos/modules/services/hardware/nvidia-optimus.nix

···
       "nvidia"
       "nvidiafb"
       "nvidia-drm"
+      "nvidia-uvm"
       "nvidia-modeset"
     ];
     boot.kernelModules = [ "bbswitch" ];